The data release includes data from four studies: (1) toxicity of a permitted effluent, which entered the Deep Fork River (DFR), Oklahoma, USA, to a unionid mussel (Lampsilis siliquoidea) and to 2 standard test species (cladoceran Ceriodaphnia dubia; and fathead minnow Pimephales promelas) in short-term 7-d effluent tests; (2) relative sensitivities of the 3 species to potassium (K), an elevated major ion in the effluent, using 7-d toxicity tests with KCl spiked into a DFR upstream reference water; (3) potential influences of background water characteristics on the acute K toxicity to the mussel (96-h exposures) and cladoceran (48-h exposure) in 4 reconstituted waters that mimicked the hardness and ionic composition ranges of the DFR; and (4) potential influence of temperature on acute K toxicity to the mussel. Water quality, survival, growth, and reproduction endpoints are reported.